A seismic survey represents an attempt to image or map the subsurface of the earth by sending sound energy down into the ground and recording the “echoes” that return from the rock layers below. The source of the down-going sound energy might come, for example, from explosions or seismic vibrators on land, or air guns or vibrators in marine environments. During a seismic survey, the energy source is placed at various locations near the surface of the earth above a geologic structure of interest. Each time the source is activated, it generates a seismic signal that travels downward through the earth. “Echoes” of that signal are then recorded at a great many locations, such as on the surface. Multiple source/recording combinations are then combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2-D) seismic survey, the recording locations are generally laid out along a single line, whereas in a three dimensional (3-D) survey the recording locations are distributed across the surface in a grid pattern. In simplest terms, a 2-D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3-D survey produces a data “cube” or volume that is, at least conceptually, a 3-D picture of the subsurface that lies beneath the survey area. In reality, though, both 2-D and 3-D surveys interrogate some volume of earth lying beneath the area covered by the survey. Finally, a 4-D (or time-lapse) survey is one that is recorded over the same area at two or more different times. Obviously, if successive images of the subsurface are compared any changes that are observed (assuming differences in the source signature, receivers, recorders, ambient noise conditions, etc., are accounted for) will be attributable to changes in the subsurface.
A seismic survey is composed of a very large number of individual seismic recordings or traces. The digital samples in seismic data traces are usually acquired at 0.002 second (2 millisecond or “ms”) intervals, although 4 millisecond and 1 millisecond sampling intervals are also common. Typical trace lengths are 5-16 seconds, which corresponds to 2500-8000 samples at a 2-millisecond interval. Conventionally each trace records one seismic source activation, so there is one trace for each live source location-receiver activation. In some instances, multiple physical sources might be activated simultaneously but the composite source signal will be referred to as a “source” herein, whether generated by one or many physical sources.
In a typical 2-D survey, there will usually be several tens of thousands of traces, whereas in a 3-D survey the number of individual traces may run into the multiple millions of traces.
Of particular interest for purposes of the instant application is obtaining accurate subsurface images from seismic data that have been acquired where there are rapidly spatially varying subsurface velocities. Rapid lateral velocity variations (as compared with the velocities in the surrounding layers) are regularly seen in some regions of the world. Of substantial economic value are exploration targets that are adjacent to or below salt structures, which are well known sources of velocity problems. Since salt structures are frequently exploration targets this is a problem that is encountered to one degree or another with some regularity. In such areas, migrating seismic data to image the salt dome and the surrounding/deeper layers using conventionally obtained velocities often produces an image where there are regions in the processed data that appear disrupted or distorted due to inaccurate velocities.
Conventional velocity estimates are obtained by having an interpreter pick (manually or with the help of autotrackers) the top of the velocity anomaly. In the case of a salt structure, after the top is picked the salt velocity is “flooded” below it and then the base of salt is interpreted/picked. Subsequent migration of the seismic data using the estimated top and bottom then provides an accurate image of the extent and thickness of the salt, and geological structures below the salt, so long as the picks are accurate. If they are not, the image below the salt can be distorted.
Thus, what is needed is a way to more accurately identify the onset of a velocity anomaly in the subsurface. Further, it would be desirable that the method requires less human intervention than has heretofore been the case.
As is well known in the seismic acquisition and processing arts, there has been a need for a system and method that provides a better way to migrate data that have been acquired over a subsurface where there are short-period velocity variations. Accordingly, it should now be recognized, as was recognized by the present inventors, that there exists, and has existed for some time, a very real need for a method of seismic data processing that would address and solve the above-described problems.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.